GB2174109A - Soldering flux - Google Patents

Abstract

A halide based aqueous flux for soldering with zinc-based solder alloys contains 0.5-6% by weight of its solid content nickel chloride and 0-10% by weight stannous chloride, the balance being selected from zinc chloride, ammonium chloride, and halides of ammonium and of an alkali metal.

Description

SPECIFICATION An improved soldering flux Traditional soft solders constituted by a lead-tin alloy have melting points up to about 2500 and among other applications are used for joining the components of automobile radiators, whereas brazes and silver solders have a far higher melting point of the order of 60000. The fact that the cost of zinc is far lower than that of silver makes the successful development of zinc-based solder alloys an attractive economic objective. An additional impetus to the development of soldering systems using zinc alloys is the possibility of replacing high-temperature solders containing silver for joints whose maximum temperature in use is below the melting-point of the zinc-based alloys used.All solder alloys must be used in conjunction with a flux, the functions of which are to facilitate wetting by the solder of the substrate metal to be soldered, to remove oxide film from the surface of the molten solder and to prepare the surface of the substrate metal. The efficiency of the flux may be tested by two tests referred to below, namely the spreading test which indicates its effectiveness in performing the first two of these functions and the capillary test which indicates its effectiveness in performing the third function. Such alloys require specific fluxing systems to achieve the combined purposes of cleaning the surface of the substrate metal, protecting it from further oxidation and promoting the wetting of the surface by the solder alloy.

Fluxes used for tin-lead solder alloys consist of aqueous pastes or solutions of zinc chloride, together with ammonium chloride, to improve the cleaning function, and sometimes ammonium or an alkali-metal halide which reduces the melting-point, and increases the fluidity of the molten zinc chloride remaining after the water has been driven off.

Zinc, as such, cannot successfully be used as a solder but zinc-based alloys having a melting point in the range of 400-500 C have been developed for use in soldering. These alloys, of which examples are given below, consist predominantly of zinc, with addition of a small proportion of an alloying element such as tin or copper and sometimes a still smaller proportion of lead.

The above-mentioned fluxes used for tin-lead solder alloys do not give satisfactory results with zinc-based solder alloys and the present invention is based on the discovery that addition to these known fluxes and similar halide fluxes of nickel chloride, preferably incorporated in the flux as a hexahydrate, and/or stannous chloride improves the wetting and spreading of these zinc-based alloys to an extent sufficient to permit of successful use for soldering.

The invention accordingly provides a method of soldering with a zinc-based solder alloy having a melting point in the range of 400-500 C, which involves the use of an aqueous flux, the solid content of which is of the following composition by weight:- 0.5-6% nickel chloride, 0-10% stannous chloride, the balance being selected from zinc chloride, ammonium chloride and halides of ammonium and of an alkali-metal.

The fluxes according to the invention are applied to the work as an aqueous solution or paste, which will normally contain at least 30% by weight of solids and preferably resembles a finely ground slurry.

The main criterion which we have used for evaluating flux compositions according to the invention has been the "coupon" or spreading test in which a known weight of solder (0.2g) and of flux (0.5g) is placed on a 3 cm square piece of substrate sheet and the whole heated to above the melting point of the solder. The area and appearance of the patch of solder formed are used to rank the solder system in effectiveness.

An additional criterion is the "capillary run" test in which a strip of substrate about 5 x 1 cm, bent at an obtuse angle, is stood on its edge on a 5 x 5 cm horizontal specimen of the same substrate. A sample (0.3g) of solder is placed at one end of the strip, while the welt or line of contact between the two pieces of sheet is filled with flux. On the application of heat the better flux systems allow the solder to run the whole length of the welt and provide a bond of subjectively adequate strength.

The following flux compositions have been investigated, and compared with a standard flux used with tin-lead solder alloys and identified as H: H : ZnCI2 42%, NH4CI 6.3%, KCI 34%, LiCI 17.7%, all in proportions by weight; HV3 : H plus 5% of its weight of nickel chloride hexahydrate; HV7 : H plus 5% of its weight each of nickel chloride hexahydrate and stannous chloride; HV8 : H plus 2.5% of its weight of nickel chloride hexahydrate; X3 :A composition omitting zinc chloride and incorporating nickel chloride hexahydrate, and of the following composition: 5% NiCl2.6H2O, 8% NH4Cl, 56.6% KCI 30.4% LiCI HV13 ZnCI240.5%, NH4Cl 8.2%, KCI 34.2%, LiCI 17.8% NiCl2.6H2O 1.5% C4 ZnCI2 57.34%, NH4Cl 30.85%, NiCl2.6H2O 7.5%, H2O 10.31% 0.2g of solder was used for the capillary run test in Table 1 below and 0.3g of solder for the other tests.

Except in the case of 04, the above formulation of which includes the necessary water, 17% by weight of water was added to the dry flux composition to make a finely ground slurry.

Tested on copper and brass substrates by both the above tests the addition of the nickel chloride showed a marked increase in the area covered during the coupon test (compositions HV3, HV8) and the additional presence of stannous chloride (HV7) improved the evenness of the solder layer so formed. The solder alloys used were as follows: Alloy 1 - 5% tin, 0.5% lead, balance zinc.

Alloy 2- 5% copper, balance zinc.

For example, on a brass substrate and alloy 1 the maximum dimension of the solder layer was: with flux'H' 12 mm with flux 'HV7' - 20 mm on brass with alloy 2 in the same test the results were:- with flux 'H' - two separate ovals, major axes 10 mm and 8 mm with flux 'HV7' - 19 mm.

The full test results are set out in the following Tables. It will be noted that all the fluxes according to the invention gave the maximum possible value of 50 mm capillary run.

TABLE 1 Results on Copper with Alloy 1 Flux Coupon test Capillary run test maximum dimension H 16mm 35mm HV3 25 mm 50 mm HV7 25 mm 50 mm HV8 25 mm 50 mm X3 22 mm 50 mm HV13 22 mm 50 mm C4 25 mm 50 mm TABLE 2 Results on Copper with Alloy 2 Flux Coupon test Capillary run test maximum dimension H 10mm 25mm HV3 21 mm 50mm HV7 23 mm 50 mm HV8 20 mm 50 mm X3 16mm 50mm HV13 20 mm 50 mm C4 23mm 50mm TABLE 3 Results on Brass with Alloy 1 Flux Coupon test Capillary run test maximum dimension H 12mm 10mm HV3 23 mm 50 mm HV7 20 mm 50 mm HV8 19 mum 50mm X3 18mm 50mm HV13 20 mm 50 mm C4 23 mm 50 mm TABLE 4 Results on Brass with Alloy 2 Flux Coupon test Capillary run test maximum dimension H 9mm 10mm HV3 19 mm 50 mm HV7 19 mm 50 mm HV8 18 mm 50 mm X3 17mm 50mm HV13 18 mm 50 mm C4 19mm 50mm A capillary run test on mild steel using alloy 1 and flux HV3 gave a strong bond over 30 mm of a 35 mm total length.

Claims (4)

1. A method of soldering with a zinc-based solder alloy having a melting point in the range of 400-500 C, which involves the use of an aqueous flux the solid content of which is of the following composition by weight: 0.5-6% nickel chloride, 010% stannous chloride, the balance being selected from zinc chloride, ammonium chloride, and halides of ammonium and of an alkali metal.

2. A method according to claim 1, wherein the nickel chloride is incorporated in the flux as a hexahydrate.

3. A method according to claim 2, wherein the flux is of the composition hereinbefore identified as HV3, HV7, HV8, HV13, X3 or C4.

4. A method of soldering substantially as described herein.